Reciprocating screen with material positioning elements

ABSTRACT

A screen arrangement for testing chip samples comprising a reciprocating frame and screen boxes fixed thereto has the screen boxes provided with screen bottoms of parallel rods or similar elements and control elements located at an angle to said rods to orientate waste materials such as sticks mixed with the chips at an angle, particularly at right angles to said rods to improve the separation of the sticks from the chips. The control elements can also consist of rods mounted in suitable spaced relation to one another.

United States Patent 1191 Ljungqvist Apr. 16, 1974 RECIPROCATING SCREEN WITH MATERIAL POSITIONING ELEMENTS [75] Inventor: Karl Johan Ljungqvist, Skoghall,

Sweden [73] Assignee: C. J. Wennberg AB, Karlstad,

Sweden 22 Filed: May 27,1971

21 Appl. No.: 147,437

[30] Foreign Application Priority Data 1,334,761 3/1920 Juhlke 209/264 1,614,586 1/1927 Anderson.... 209/264 X 1,731,115 [0/1929 Taylor 209/264 X 2,139,046 12/1939 Thys 209/267 X 4,080 6/1845 Hizer 209/393 X 342,392 5/1886 Scott 209/393 FOREIGN PATENTS OR APPLICATIONS 848,184 7/1952 Germany 209/395 939,075 9/1962 Great Britain 209/12 304,395 9/1968 Sweden 209/315 636,143 8/1934 Germany 209/393 Primary Examiner-Robert Halper Attorney, Agent, or Firm-Young and Thompson 5 7] ABSTRACT A screen arrangement for testing chip samples comprising a reciprocating frame and screen boxes fixed thereto has the screen boxes provided with screen bottoms of parallel rods or similar elements and control elements located at an angle to said rods to orientate waste materials such as sticks mixed with the chips at an angle, particularly at right angles to said rods to improve the separation of the sticks from the chips. The control elements can also consist of rods mounted in suitable spaced relation to one another.

4 Claims, 19 Drawing Figures PATENTEDAPR 1 61914 804,246

SHEET. 1 0F 3 INVENTOR. KARL Jo /w Luwvewsr IQTTYJ.

Pmimmmw 3.804246 sum 3 0F 3 FIGCH INVENTOR. 144 mm day/11v L uu/ve wsr BY raw) RECIPROCATING SCREEN WITI-I MATERIAL POSITIONING ELEMENTS the chips piece by piece in a representative sample, and

usually chip thickness and sometimes chip length and width are measured. As a representative sample must be of some size, such a measurement by hand is in practice hardly possible. A sufficiently large sample is bet ter examined in an analytical chip screen.

At present two methods for analytical screening are known. In the first a screen bottom with round, square, or rhombic holes is used. The sample of chips is divided into two fractions in a way that is not well defined. In the second method a screen with a slot bottom is used. The bottom of the screen consists of parallel rods placed at a constant distance, forming slots. Pieces of chips as thick as, or thinner than, the distance between the parallel rods can pass through the bottom. By this screening the chip sample is divided into two fractions, one of which contains the chips thicker than the slots and the other, the thin chips.

The rods may be placed at the same height in a screen bottom forming a plane. However, such screen bottoms do not work well. The chip pieces rest on two or several rods and during the screening they slide to and fro along the screen bottom. No forces work on the chips to cause them to stand on end, and thus the chips can not pass through the bottom. According to the Swedish patent 304,395 these disadvantages can, however, be eliminated by placing the slot rods at different heights in relation to each other.

The analytic method according to the Swedish patent 304,395 has a technicaland economical value in that the yield of screened pulp at a given pulping degree can be calculated. The homogenity of pulp can be estimated as well.

The calculation of screenings after sulphate pulping is demonstrated as follows:

Chips are divided in thickness fractions by screening in a slot screen. Suitable intervals of the slot widths are shown in Table I.

TABLE 1 Fraction 1 thinner than 2 mm Fraction 2 thicker than 2 mm, but thinner than 4 mm Fraction 3 thicker than 4 mm but thinner than 6 Fraction 4 thicker than 6 mm but thinner than 8 Fraction thicker than 8 mm but thinner than 10 mm Fraction 6 thicker than 10 mm but thinner than l2 mm Fraction 7-thicl er than 12 mm The quantity of pulp from a given quantity of wood, the yield. is influenced by a great number of factors, such as the cooking chemicals. the pulping degree. the kind of wood and the pulping scheme. In order to give a simple description of the effect obtained by the invention, it is assumed that the above factors are kept constant. Only the chip size, and the amount of cracks in the chips affect the yield.

The weight of fraction 1, M l, is multiplied by a factor K 1. In this way the weight of screened pulp from fraction 1, here called U l, is calculated. The weight of fraction 2, M 2, is multiplied by another factor K 2, and the amount of screened first-class pulp from fraction 2 is obtained. The numerical value of the K-factors depends in the main on the chip thickness and the cracks in the wood.

There are, however, a number of cracks in technical chips which seems to facilitate the penetration of chips in the impregnation part of pulping. The number and character of micro-cracks in the chips vary with different types of chippers which change the K-value. However, the variation of K between the conventional chippers is small.

When the chipper is not correctly adjusted, however, the last part of the log turns over and is cut or split parallel to the fibre direction. Thus big, thick, and long chips are formed, here called sticks. Sticks are also often formed when short logs and sawmill waster are chipped. The quantity of sticks can be considerable.

Sticks are usually considered as a contamination of the chips, as they can cause plugging of technical screens, bridging in chip silos and circulation troubles in the digesters.

The sticks do not have any micro-cracks. This is an important disadvantage.

The pulp yield of chips is higher than the pulp yield of sticks for the same thickness. This is shown in Table 2, where the quantity of screenings is compared afte sulphate pulping to chlorine number 7.

TABLE 2 Sulphate pulp chlorine number 7. Screenings in percent of wood, value of K. Yield coefficient K Fraction Slot width in pulp K in pulp K No. mm from from chips sticks Purchased chips will obviously be classified in a higher quality group than the real one if the chips contain sticks and a sample of the chips is classified in a slot screen. Correctly classified, the sticks should be completely separated from the chip sample or alternatively collected inn those thickness fractions by the screen, where the K-value of the sticks is the same as the K-value of the chips.

Surprisingly this has now been found possible by completing the slot screen according to the Swedish patent 304,395 with slot elements of a new type, here called control elements, which are placed in the bottom plane of the screen, perpendicular or at oblique angles to the slot elements.

The effect of the control elements can be explained in the following way: Characteristic of a stick is the dominating length and a usually moderate width and thickness. As long as a stick is orientated in the bottom plane with its longitudinal shaft at an angle to the slot elements it can not pass the slot bottom. The forces upon movement of the screen are too small to raise the sticks in a vertical direction. During the to and fro motion of the screen the sticks will, however, in a short time be orientated in the bottom plane parallel to the slot rods. In such a position they can easily pass the slots if they are thin enough.

If, however, control elements are placed above the screen bottom, in contact with or close above the slot rods directed along the path of screen motion, perpendicular to the slots, the sticks will be orientated in a direction parallel to the control elements. This orientation force will be stronger the closer the control elements are placed. Thus an increasing part of the sticks is prevented from passing through the bottom.

The pieces of chips, on the other hand, are not influenced by the control elements as long as the distance between two consecutive control elements is larger than the width or length of the chip pieces to pass. Two consecutive control elements should be placed at a distance of at least 15 mm in a screen bottom for small and thin chips, and at least 30 mm in a bottom for big and thick chips.

Thus the result will be that the chip pieces pass a slot bottom provided with control elements in the same way as at normal slot screening, while 50 percent or more of the sticks with the same thickness will be left on the bottom. These sticks will pass the bottom if the control elements are removed. Those sticks that pass a slot bottom equipped with control elements are the short and thin ones in the actual thickness fraction, and those which can not pass are the long and thick ones. In this way the sticks are adequately classified.

In Table 3 it is shown how an increasing part of the sticks, all of them thinner than the slot width of the screen bottom, remain on the slot bottom when the number of control elements in the bottom is increased.

TABLE 3 Part of sticks. in percentage by weight of charged quantity of sticks (1,100 g) thinner than 12 mm and partly thinner than l and 8 mm, which do not pass the slot bottom when the number of control elements is increased.

Slot Thickness of Number of control elements per I00 mm width the sticks 2 4 mm mm Percentage by weight of sticks l2 12 ll 2 I4 26 30 It) 0 4 I) 31 33 8 8 0 4 I6 22 24 TABLE 4 Slot bottoms without control elements compared with bottoms provided with four control elements per I00 mm.

Quantity of sticks Part of sticks that do not pass because of the control elements Slot width weight thickness Number of control elements mm grams mm ()IIOU mm 4lltlt) mm Percentage by wt. of sticks I40 l2-l0 O 58 I2 230 lO-S 27 230 l0- 8 0 52 I0 545 8- 2 0 3| 8 545 8- 2 O 52 The effect of screening with control elements is further elucidated by the fact that the sticks belonging to the fraction 8 2 mm (545 g) which did not pass the 8 mm slot bottom provided with control elements had an average thickness of 7 mm, while the sticks that passed tha bottom has an average thickness of 5 mm.

In principle it is practical, although not necessary to orientate the control elements exactly perpendicularly to the slot direction; an oblique orientation is also efficient.

If it is wanted to isolate a larger part of the sticks, belonging to the thickness fraction, than shown in Tables 3 and 4, the control elements should be placed under the slot bottom. In this case the sticks are orientated in the direction of the slots, but they can not pass the control elements. The effect is more pronounced when the control elements are placed closer together.

The combination of control elements located above or under the slot bottom at various distances from each other offers various possibilities to isolate sticks of different sizes and shapes from chips, or to direct them to the thickness fraction where they are correctly classified.

The invention will be further explained in the following description of embodiments illustrated diagrammatically in the accompanying drawings in which FIG. 1 is a vertical elevation partly in section of a screen arrangement seen towards one of the long sides of the screens.

FIG. 2 is a top view ofthe arrangement shown in FIG.

'FIG. 3 is a vertical end view of the same arrangement,

FIG. 4 is a vertical cross section taken on line 4-4 in FIG. 3 and showing superimposed slot bottoms,

FIG. 5-8 illustrate a modified embodiment in the same manner as in FIG. I-4, and

FIG. 5 is a vertical elevation,

FIG. 6 is a top view,

FIG. 7 is a vertical end view,

FIG. 8 is a vertical cross section taken on line 8-8 in FIG. 7,

FIG. 8a is a fragmentary cross section of the topmost slot bottom shown in FIG. 8 but in an enlarged scale,

FIGS. 12-15 are vertical sections on an enlarged I scale illustrating various positions of control elements relative to the slot bottom elements.

In the two embodiments of the invention shown in FIG. 1-4 and 5-8, respectively, the common screen arrangement is similar. There is a stand 1 on a bottom plate 2, a movable frame 3 hinged on horizontal pivots 4 near the top of the stand 1 by links 5 connected to the ends of the movable frame 3 by pins 6 to be reciprocated by a driving unit 7. The unit 7 has an eccentric disc 8 cooperating with a connecting rod 9 which is hinged to the movable frame 3 at 10, part of the stand being broken out to show said hinged connection in FIG. 2 and 6. A number of interchangeable screen boxes I1, 12 and l4'are superimposed on and supported by the shaking frame 3. The screen boxes are fixed to one another and to the frame 3 by steering pins 15 and thumb screw units 16. A collecting box 17 is located beneath the hinged frame 3. The screen boxes 11, 12, 14 include slot bottoms 18, 19 and 20 respectively, formed by'rods 18a, 19a and 20a respectively.

in each bottom mounted in one or more planes, and control elements in the form of rods 21 at an angle to the slot bottom rods as more particularly illustrated in FIGS. 11-14.

In the arrangements of rods shown in FIG. 4 and FIG. 8 and, more particularly, in FIGS. 8a, 8b, and 8c, the slot bottoms have their rods positioned in two planes. In FIG. 8a there are in a top plane two rods 18a opposite the space between groups of three rods in a bottom plane, in FIG. 812 only one rod 19a in the top plane between groups of three rods in the lower plane,

and in FIG. 8c one rod 20a in the top plane between groups of two rods in the bottom plane.

FIG. 9, l0 and a illustrate further arrangements of slot bottom rods in one plane and in two planes, the diameter d of the rods and the distance s therebetween (slot width) being indicated. In FIG. 10a the rods are indicated as positioned in the apices of triangle, the apex angle of which is v.

The top view according to FIG. 11 indicates parallel control rods mounted to form right angles with parallel slot bottom rods 18 which may be arranged in any of the manners shown in FIGS. 8-10a inclusive. FIG. 12 illustrates a part of an embodiment in which the control rods 21 are located above, FIG. 13 underneath and PIC-.14 both above and beneath the slot bottom elements. FIG. illustrates an arrangement in which a control element comprises rods, strips or similar means 22 having its top line spaced from the tops of the slot bottom elements, the control elements extending from said top line to a bottom line below the lowest slot bottom elements 18.

The embodiment shown in FIGS. 5-8 as compared with the one illustrated in FIGS. l-4 is completed with accessory devices for continuous screening such as an oblique guide plate 23 in each box supported at its lower end by a horizontal rod 24 and secured at its upper end to one end wall 25 of the box. This end wall has an outlet beneath the plate 23 through which the reject can escape through a spout 25.

When screening in batches (FIGS. l 4) the sample is supplied to the top screen box 14 wherein the thickest chip fraction will remain. The chips are collected in the lower boxes all according to thickness. All chip material smaller than the slot width of the lowest screen box 11 is collected in the collection box. For further examination of the chip fractions the screen boxes 11, 12, 14 (FIGS. I-4) and the collecting box 17 may be emptied manually.

When screening continuously (FIGS. 5-8) the chip fractions escape continuously through the spouts 26, the thickest chips through the topmost spout, and can be examined at any time during the screening procedure. The operation of the control elements has been explained hereinbefore.

It is to be noted that the control elements need not be fitted in direct contact with the slot elements; a short distance to the slot elements does not impair the function thereof.

What I claim is:

1. A screen arrangement for dividing chip samples containing sticks into thickness fractions and simultaneous separation of the sticks, comprising a stand, a frame suspended from said stand for reciprocation therein in a horizontal direction, means for reciprocat ing said frame in said horizontal direction, screen boxes fixed to said frame and to one another to move together with said frame, each box including a screen bottom of elongated parallel closely spaced horizontal slotforming elements disposed at a right angle to said direction and extending across each said box in planes at different levels, and elongated parallel horizontal control elements positioned at a right angle to said slot-forming elements and extending across each said box at mutual distances greater than the spaces between said slotforming elements.

2. A screen arrangement as claimed in claim 1, said control elements being disposed above said slotforming elements.

3. A screen arrangement as claimed in claim 1, said control elements being disposed below said slotforming elements.

4. A screen arrangement as claimed in claim 1, said slots being 2 to 15 mm. wide and said mutual distances between said control elements being 25 to I00 mm. 

1. A screen arrangement for dividing chip samples containing sticks into thickness fractions and simultaneous separation of the sticks, comprising a stand, a frame suspended from said stand for reciprocation therein in a horizontal direction, means for reciprocating said frame in said horizontal direction, screen boxes fixed to said frame and to one another to move together with said frame, each box including a screen bottom of elongated parallel closely spaced horizontal slot-forming elements disposed at a right angle to said direction and extending across each said box in planes at different levels, and elongated parallel horizontal control elements positioned at a right angle to said slot-forming elements and extending across each said box at mutual distances greater than the spaces between said slotforming elements.
 2. A screen arrangement as claimed in claim 1, said control elements being disposed above said slot-forming elements.
 3. A screen arrangement as claimed in claim 1, said control elements being disposed below said slot-forming elements.
 4. A screen arrangement as claimed in claim 1, said slots being 2 to 15 mm. wide and said mutual distances between said control elements being 25 to 100 mm. 